Method of improving the corrosion resistance of oxidized metal surfaces



United States Patent Ofiice 3,351,540 Patented Nov. 7, 1967 3,351,540METHGD F IMPROVING THE CQRRO- SHQN REESTANCE 0F ()XIDIZED METAL SURFAEESCharles J. Amore, New Haven, and James F. Murphy, Hamden, (Ionm,assigners to ()iin Mathieson Chemical Corporation, a corporation ofVirginia No Drawing. Filed Mar. 23, 1964, Ser. No. 354,162 9 Claims. (1.ant-35 ABSTRACT OF THE DISCLQSURE The present invention relates to amethod of improving the corrosion resistance of oxidized metal surfaces,particularly of anodic coatings on aluminum or aluminum base alloysurfaces, by electrolytic treatment in a bath of a soluble amphipathicmaterial.

For many purposes an oxide layer is formed as a protective coating onaluminum surfaces. This is accomplished by making the aluminum the anodein an electrolytic cell having an electrolyte formed of about 2 to 70percent sulfuric acid or other acids or acid salts of the type, chromicacid, oxalic acid, sulfamic acid and the like. Any suitable metal, suchas lead, for example, may form the cathode. A voltage of about 10 to 20volts is impressed upon the cell while the electrolyte is held at asuitable temperature, such as from 10 to 50 C., while an oxide coatingof the desired thickness is formed. Generally, the time of treatmentvaries from to 60 minutes or more depending upon the thickness of thecoating desired.

In addition to the foregoing, there are a number of less commercialprocesses for producing oxide layers on aluminum or aluminum basealloys.

The oxide layer formed under these conditions consists generally ofrelatively anhydrous alumina, A1 0 containing sulfate ion. Particularlywhen formed by anodic oxidation, the layer of aluminum oxide is hard,porous, highly absorbent, and of substantial thickness, depending uponthe particular aluminum sample treated and the specific process used forforming the oxide coating.

The formation of an oxide coating on the aluminum surface is intended,inter alia, to improve corrosion resistance, to improve resistance toabrasion, and to improve absorption of coloring in order to providepermanent coloration on the aluminum surface.

It has been found heretofore that the characteristics of the oxide filmcan be markedly improved by a process generally known as sealing. Thesealing process renders the oxide film relatively impervious and lessporous, The sealing process is generally carried out commercially byimmersing the anodized aluminum article in water maintained near itsboiling point, i.e., within about F. of the boiling point. The sealingprocess is believed to result primarily in the conversion of asubstantial part of the porous and pervious oxidefilm of anhydrousalumina to a hydrated product, such as alumi num monohydrate, Al O H O,with resultant swelling or volume increase of the oxide particles topartially close or seal the pores.

This sealing process improves the resistance to corrosion significantlyand markedly. It is highly desirable, however, to obtain a still furtherimprovement in the corrosion resistance of the anodized aluminumarticle. Pursuant to this goal, numerous methods have been proposedaiming towards improving the corrosion resistance still further thanthat which is obtained by conventional sealing processes. For example,the sealing bath has been modified by various additions, such as chromicacid or boric acid or metal salts, such as nickel acetate and others.These modified processes offer some improvement but still leaveconsiderable room for still greater improvement in the corrosionresistance.

conventionally, various accelerated tests have been devised to determinethe corrosion resistance of an aluminum specimen, for example, the CASStest and a cathodic electrochemical breakdown test. These tests enableus to determine quickly and expeditiously the corrosion resistance of analuminum surface. In the CASS test, a fine spray of a solutioncontaining 58 gm. per liter sodium chloride, 0.264 gm. per liter cupricchloride, pH 3.0 adjusted with glacial acetic acid and at F., is allowedto impinge upon the aluminum article .for several hours. The sample isthen removed, cleaned, and a 4 inch grid placed over the surface of thearticle; the number of squares containing one or more pits is.determined. The ratio of squares containin one or more pits to the totalnumber of squares, multiplied by 100, gives the percent area affected.In the cathodic breakdown test the anodized aluminum article is madecathode in a cell containing an electrolyte and platinum anode. Avoltage is impressed across the cell and associated circuitry, whichresults in current flow through the oxide. The resultant voltage dropacross the cell is measured and the integral of the voltage-time curveover a 3-minute interval determined. The integrated value involt-seconds is the test number.

An additional disadvantage of conventional sealing practices, forexample, sealing in boiling water or in nickel acetate solution, is thatthey generally lead to changes in the susceptibility of the anodiccoating to crazing on either bending or thermal expansion.

It is, therefore, the principal object of the present invention toprovide a method for improving corrosion resistance of oxidized metalsurfaces, and particularly of an oxide coated aluminum surface.

It is a still further object of the present invention to provide aprocess as aforesaid which simply, effectively, and expeditiouslyattains an improved level of corrosion resistance heretoforeunattainable.

It is a still further object of the present invention to obtain a highlycorrosion resistant oxidized metal article and especially an oxidecovered aluminum or aluminum alloy article.

It is an additional object of the present invention to provide a processfor obtaining a highly corrosion resistant article as aforesaid which inaddition will not craze easily as do conventionally sealed articles.

Further objects and advantages of the present invention will appearhereinafter.

In accordance with the present invention, is has now been found that theforegoing objects and advantages may be readily accomplished and amethod provided for improving the corrosion resistanct of oxidized metalsurfaces and particularly of an anodized oxide coating formed onaluminum or aluminum base alloys.

The foregoing objects are obtained by (l) immersing the oxide coatedmetal in a bath of a soluble amphipathic material, preferably in a bathof a fatty acid salt; (2) passing an electric current of at least 2volts through said bath between an inert cathode and said oxide coatedsaid amphipathic material in an amount from 0.0001

percent to saturation.

The improved article of the present invention is a metal articlecontaining an oxide coating thereon of at least 0.01 mil -in thickness,at least the major portion of said oxide coating, including the surfacesof the pores, containing adsorbed therein an amphipathic material.

The process of the present invention provides a simple method forimproving the corrosion resistance of oxidized metal surfaces andparticularly of an oxide coated aluminum surface and provides a highlycorrosion resistant oxide covered article. In addition, the presentprocess produces an article which will not craze easily, as doconventionally sealed coatings.

When an oxide coated metal is simply immersed in a solution of anamphipathic material, only the outer, easily accessible surfaces aretreated. Thus, the barrier to corrosive reaction is considerably lessthan in accordance with the present invention wherein at least the majorportion of the oxide coating, including the surfaces of the pores,contains the amphipathic material adsorbed therein. While it is believedthat in accordance with the present invention the amphipathic materialis adsorbed in the oxide coating, the present invention is not intendedto preclude the possibility that some or even considerable reaction isoccurring in the oxide.

An additional, surprising feature of the present invention is thatamphipathic materials are wetting agents and in accordance with thepresent invention we are obtaining a hydrophobic or nonwetting surfaceby treatment with a wetting agent.

The process of the present invention is readily applicable to anyaluminum or aluminum base alloy in accordance with prior practices whichcontains sufficient aluminum to anodize in the conventional fashion toproduce a reasonably thick anodic coating. This would include all of thestandard and non-standard aluminum base alloys. Exemplificative aluminumbase alloys which may be readily employed include but are not limited toaluminum alloys 1100, 3003, 5453, 5053, 5252, 6061 and 60 63. Moregenerally, the present invention is applicable to any oxidized metalsurface, the metal of which forms an insoluble compound with thetreating solution, for example, magnesium, iron, copper, zinc and alloysthereof.

In accordance with the present invention, the oxide coated aluminum, forexample, is immersed in a bath of a soluble amphipathic material (fattyacid salt, for example) and preferably a solution containing saidmaterial in an amount of from 0.0001 to saturation. The bath may be, forexample, the molten amphipathic material provided that the pure materialis stable under the treatment conditions. In the preferred embodiment ofthe present invention, a solution of the material is used and the majorportion of the solvent is water for reasons of economy; however, all orpart of the solvent may include water miscible organic materials, e.g.,isopropyl alcohol, ethylene glycol, glyce-rine, dirnethyl sulfoxide,etc. The concentration of the material in solution is generally in therange of from 0.0001 to 20 percent by weight, preferably from 0.001 topercent by weight and optimally in the range of 0.01 to 1.0 percent.

Any soluble amphipathic material may be used in the process of thepresent invention. The term amphipathic refers to ions which are highlypolar and water soluble in one portion of the molecule and non-polar andwater insoluble in another. The preferred materials are the watersoluble fatty acid salts saturated or unsaturated, i.e., fattycarboxylates, especially the long chain fatty acids which contain one ormore unsaturated bonds. Typical fatty acid salts include lithium,sodium, potassium, cesium salts of oleic, stearic, palmitic, erucic,ricinoleic, pelargonic, suberic, azeleic and lauric acids, and so forth.Other amphipathic materials include, but are not limited to anionicsurfactants, such as alkyl or aryl or alkylaryl sulfonates,phosphonates, sulfates and phosphates. In addition, sulfonated fattyacid salts, cationic surfactants, such as primary, secondary or tertiaryfatty amines, primary, secondary or tertiary fatty aminic salts andfatty quaternary aminic salts, and soforth.

As the anodized oxide coated article is immersed in the bath, anelectric current of at least 2 volts anodic potential is passed throughthe bath between an inert cathode and the oxide coated aluminum asanode. During the electric treatment the bath is maintained at atemperature of between about 50 and 150 C. The preferred temperature is70 to C. since normally water will be a major portion of the solvent.Naturally, inert materials may be added to the bath to either raise ordepress the boiling point of the bath depending on treatmentrequirements.

The treatment time is not especially critical and generally will dependon other variables such as current, particular alloy, temperature, etc.Generally, however, the treatment should be maintained for at leastthirty seconds and preferably not over 60 minutes.

An electric current of at least 2 volts anodic potential is passedthrough the bath as aforesaid. A minimum of at least 2 volts anodicpotential is required; but the upper voltage limit is not especiallycritical. It is preferred, however, for convenience of handling that avoltage of less than 150 volts be employed and optimally a voltage inthe range of from 70 to volts.

The electric current is passed through the bath between an inert cathodeand the oxide coated metal as anode. The particular cathode employed isnot especially critical other than the requirement that the cathode besubstanial- 1y inert. For example, the alloy being treated may be usedas cathode or stainless steel, lead, mild steel, carbon, aluminum andits alloys, or, in general, any inert material which does not adddeleterious cations to the solution.

In the preferred embodiment the sample is sealed subsequent to thetreatment of the present invention. The sample may be sealed prior tothe treatment but this increases the crazability of the coating. Thesealing is accomplished by immersing the sample in water or an aqueoussolution maintained within about 10 F. of its boiling point andpreferably boiling water. Preferably, very short sealing times, such as2 minutes or less are employed.

The improved article of the present invention contains an oxide coatingthereon of at least 0.01 mil in thickness, the major portion of saidoxide coating, including the sur faces of the pores, containing adsorbedtherein an amphipathic material. The improved article of the presentinvention has numerous advantages over conventional materials, forexample, it is non-staining with organic dyes or other materials. Thesurface is hydrophobic in depth and does not craze under thermal cyclingor thermal shock. The surface exhibits a lower friction thanconventionally sealed surfaces and exhibits very low absorption of ionicmaterials. In addition, the article of the present invention shows avery high corrosion resistance and a very high resistance to cathodicattack.

A modification of the present invention includes pretreatment with ametal hydroxide solution, e.g., ammonium hydroxide, of a pH of at least8 at a temperature of from 0 to 100 C., i.e., immersion in such asolution for at least thirty seconds.

The present invention and improvements resulting therefrom will be morereadily apparent from a consideration of the following illustrativeexamples.

EXAMPLE I A popularity of 4 x 6 inch panels of aluminum alloy 5457 wereanodized for 20 minutes at 12 amps./ sq. ft. and at 22 C. This treatmentformed an anodized oxide coating on the aluminum samples of about 0.3mil in thick ness.

Four of such samples were sealed in boiling distilled water for 15minutes. Four additional samples were sealed in boiling water for 15minutes and subsequently treated in accordance with the treatment of thepresent invention as follows: the oxide coated aluminum samples wereimmersed in a dilute aqueous solution containing 0.1 percent by weightof sodium stearate; the bath was maintained at 100 C. and an electriccurrent was passed 5 through said bath between an inert cathode ofunanodized aluminum alloy 5457 and the oxide coated aluminum as anode atan anodic potential of 30 volts for about 5 minutes.

A third set of four anodized oxide coated samples was treated in thesame manner as the second set except that the third set of four sampleswas not sealed and the current was passed through the dilute solutionfor 15 minutes.

All samples were CASS tested for two 16-hour periods and the percentagearea affected by pitting corrosion on each was determined in the mannerpreviously described. The results of this evaluation are shown in thefollowing table:

EXAMPLE II A water solution containing 1 percent sodium stearate wasprepared and heated to 100 C. in a 4 liter beaker. A cylindrical sheetof aluminum alloy 5457 was placed in the beaker as cathode. The naturalpH of the solution was 9.5.

Several 4 X 4 inch panels of aluminum alloy 5252 were anodized in 18percent sulfuric acid solution at 22 C. for 20 minutes. The currentdensity was maintained at 12 amps per square foot; this treatment formedan oxide coating on the aluminum surface of about .0003 inch inthickness.

Two samples, identified as Samples A, were sealed in boiling water for15 minutes at 100 C.

Two additional samples, identified as Samples B, were sealed indistilled water for 5 minutes and then immersed in the sodium stearatesolution as anode and a current passed through the solution under a 100volts D.C. ptential for 2 minutes at 100 C.

Two additional samples, identified as Samples C, were immersed in awater solution of ammonium hydroxide for 2 minutes and then anodicallytreated in the sodium stearate bath at 100 volts for 2 minutes at 100 C.

Two additional samples, identified as Samples D, were immersed directlyin the stearate solution and anodically treated at 100 volts at 100 C.

Finally, two additional samples, identified as Samples E, were immersedin the ammonium hydroxide solution for 2 minutes and then anodicallytreated for 2 minutes at 60 volts at 100 C. in the sodium stearate bath.

All panels were CASS treated for 16 hours and the amount of pittingcorrosion determined in the manner previously described. The results areshown in the following table:

TABLE II Treatment: Percent area affected Samples A-15 minute seal 41Samples B- minute seal, 2 minutes, 100 v.

Samples C-2 minutes NH OH-pH 11.5, 2 minutes, 100 v.l00 C. Samples E--2minutes NH.;OH--pH 11.5, 2 minutes, 60 v.100 C 9 Samples D--2 minutes,100 v.--100 C.

All of the treated samples, except the conventional sealed Samples A,resisted crazing upon bending, or by thermal shock at 150 C.conventional sealed Samples A crazed badly under these conditions.

6 EXAMPLE 111' Two l-inch diameter disks from each treatment in ExampleII were immersed in a 0.1 percent sodium chloride solution for 64 hours.The chloride ion was tagged with the radioactive isotope of chlorine 36so that the amount of the chloride adsorbed could be determined.

The chloride adsorbed by the anodic coating was greatly reduced by themethod of the present invention as evidenced by the following data shownin the table below:

TABLE III Amount Treatment: of Cladsorbed Samples A-15 minute seal 34.0,ug./ sq.

inch. Samples B-15 minute seal, 2 minutes, v.-100 C 10.4 ,ug/sq.

inch. Samples C2 minutes NH OH pH 11.5, 2 minutes, 100 v.-100 C. 1.4,ug./sq.

inch. Samples E-2 minutes NH OH- pH 11.5, 2 minutes, 60 v.-l00 C. 0.9,ug/sq.

inch. Samples D-- 2 minutes, 100 v.100 C 0.6 ig/sq.

inch.

EXAMPLE IV In the cathodic breakdown test described above, a 0.3 milanodic film sealed in the conventional manner by immersion in boilingwater for 15 minutes gave an ampere second resistance of about 800.

Samples treated in accordance with the present invention gaveconsiderably higher test numbers, thus indicating improved resistance tocorrosive attack. For example, samples treated in a manner after SamplesD in Example II in a solution containing 1 percent sodium stearate usedat 100 C. with 100 volts applied gave a number of 5300. Similarly, asample treated at only 20 volts gave a number of 1000. Voltages betweenthese two limits gave intermediate values. Similar results were obtainedover a range of concentrations of sodium stearate from about 0.01 to 3percent and over.

EXAMPLE V In a manner after Example II, the treatment of Ex ample II,Samples D was repeated utilizing instead of sodium stearate, 1 percentaqueous solutions of sodium oleate, sodium laurate and potassiumricinoleate. Similar results were obtained, with all of the samplesresisting crazing upon bending or by thermal shock.

EXAMPLE VI In a manner after Example II, the treatment of Example II,Samples D was repeated utilizing a 0.1 percent aqueous sodium lauroylsarcosinate solution under a 60 volts DC. potential for 2 minutes at 100C. After 16 hours of CASS testing, there were no signs of deteriorationof the samples. In addition all samples resisted crazing upon bending orby thermal shock.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:

1. A method of improving the corrosion resistance of oxidized aluminumsurfaces which comprises:

(A) immersing the oxide coated aluminum in an aqueous solutioncontaining from 0.0001% to saturation of a soluble organic amphipathicmaterial selected from the group consisting of a fatty acid salt, ananionic surfactant and a cationic surfactant; and

(B) passing an electric current of from 2 to 150 volts through said bathbetween an inert cathode and said oxide coated metal as an anode forfrom 30 seconds to 60 minutes, said bath being maintained at atemperature of from 50 to 150 C., to provide an oxide coating of atleast 0.01 mil in thickness, at least the major portion of said oxidecoating containing adsorbed therein said amphipathic material.

2. A method of improving the corrosion resistance of oxidized aluminumsurfaces which comprises:

(A) immersing the oxide coated aluminum in an aqueous solutioncontaining from 0.0001% to saturation of a fatty acid salt; and

(B) passing an electric current of from 2 to 150 volts through said bathfor from 30 seconds to 60 minutes between an inert cathode and saidoxide coated aluminum as an anode, said bath being maintained at atemperature of from 50 to 150 C., to provide an oxide coating of atleast 0.01 mil in thickness, at least the major portion of said oxidecoating containing adsorbed therein said fatty acid salt.

3. A method according to claim 2 wherein said fatty acid salt is presentin an amount from 0.001 to 10 percent. percent.

4. A method according to claim 2 wherein said current is from 70 to 120volts.

5. A method according to claim 2 wherein said temperature is from 70 to100 C.

6. A method according to claim 2 wherein subsequent to said step (B)said oxide coated aluminum is sealed by immersion in an aqueous solutionmaintained within about 10 F. of its boiling point.

- 7. A method according to claim 2 wherein prior to said immersion theoxide coated aluminum is immersed for at least 30 seconds in a metalhydroxide solution maintained at a pH of at least 8 and a temperature offrom to 100 C.

8. A method of improving the corrosion resistance of oxidized aluminumsurfaces which comprises:

(A) immersing the oxide coated aluminum in a bath of an aqueous solutionof a fatty acid salt, said fatty acid salt being present in an amountfrom 0.001 to 10 percent;

(B) passing an electric current of from 70 to 120 volts through saidbath for from 30 seconds to minutes between an inert cathode and saidoxide coated aluminum as an anode, said bath being maintained at atemperature of from to C.; and

(C) sealing said oxide coated aluminum by immersing in an aqueoussolution maintained within about 10 F. of its boiling point, to providean oxide coating of at least 0.01 mil in thickness, at least the majorportion of said oxide coating containing adsorbed therein said fattyacid salt.

9. A method according to claim 1 wherein said soluble amphiphaticmaterial is a phosphate.

References Cited UNITED STATES PATENTS 3,120,695 2/1964 Burnharn 2925.313,210,184 10/1965 Uhlig 961 2,262,967 11/1941 Schenk 20458 2,469,2375/1949 Mason 20458 2,755,239 7/1956 Glauser 20435 2,776,918 1/1957Bersworth 1486.14 3,016,339 1/1962 Riou et al 204--38 3,026,255 3/1962Riou et a1. 204-33 3,230,162 1/1966 Gilchrist 204181' 3,265,597 8/1966Neunzig et al. 204-58 FOREIGN PATENTS 548,033 9/1942 Great Britain.656,566 2/ 1938 Germany.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

W. VAN SISE, Assistant Examiner.

1. A METHOD OF IMPROVING THE CORROSION RESISTANCE OF OXIDIZED ALUMINUMSURFACES WHICH COMPRISES: (A) IMMERSING THE OXIDE COATED ALUMINUM IN ANAQUEOUS SOLUTION CONTAINING FROM 0.0001% TO SATURATION OF A SOLUBLEORGANIC AMPHIPATHIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF AFATTY ACID SALT, AN ANIONIC SURFACTANT AND A CATIONIC SURFACTANT; AND(B) PASSING AN ELECTRIC CURRENT OF FROM 2 TO 150 VOLTS THROUGH SAID BATHBETWEEN AN INERT CATHODE AND SAID OXIDE COATED METAL AS AN ANODE FORFROM 30 SECONDS TO 60 MINUTES, SAID BATH BEING MAINTAINED AT ATEMPERATURE OF FROM 50 TO 150*C., TO PROVIDE AN OXIDE COATING OF ATLEAST 0.01 MIL IN THICKNESS, AT LEAST THE MAJOR PORTION OF SAID OXIDECOATING CONTAINING ABSORBED THEREIN SAID AMPHIPATHIC MATERIAL.